1. Field of the Invention
The invention relates to a calibration method and system, and more particularly to a high-speed calibration method and system for a scanner.
2. Description of the Prior Art
A problem prevalent in image scanning or digitizing systems is the requirement for a calibration operation in order to correct for non-uniformities therein prior to use. Generally, in beam scanning systems and plural element scanning systems, such as charge-coupled devices (CCDs), the sensor(s) must be calibrated. Calibration of a sensor offset is directed to determining the level of the signal in response to reflective or non-reflective regions of the document, for example a black region in a black-and-white document. Calibration also is directed to characterizing the gain of the sensor over a range of reflectance so as to adequately adjust any amplification of the signal to maximize the dynamic range thereof.
In systems employing plural element scanning devices, such as charge-coupled devices, for viewing by raster scanning an original, the output signal produced by the CCD includes a potential attributable to the inherent operating characteristics of the CCD. To restore the image output signal of the CCD to a true or absolute value, the potential derived from the CCD, referred to as the offset potential or signal, must be removed from the image signal. However, if the offset signal that is removed is greater or less than the actual offset signal, a noticeable aberration or distortion in the image output signal may result. Since the operating characteristics of a CCD often vary widely from one CCD to another and even vary from time to time for the same CCD or for different integration rates, the accurate determination of the offset signal to be removed is often difficult. The problem is further complicated in systems where multiple CCDs are employed.
Heretofore, various apparatus and methods have been developed to address the sensor characterization problem, some of which are described in the following disclosures which may be relevant:
U.S. Pat. No. 3,586,772 to Hardin, issued Jun. 22, 1971, discloses an optical character reader which employs a clipping level determined as a function of black and white peaks detected during a normalization scan.
U.S. Pat. No. 4,555,732 to Tuhro, issued Nov. 26, 1985, is another example of a device that corrects for offset and gain drift. Tuhro discloses an image sensor correction system which maintains the offset voltages in the shift registers of a multi-channel image sensor substantially equal. U.S. Pat. No. 4,555,732 discloses that a pair of control gates permits sampling the current offset voltages in the shift register of each channel to provide an adjusted potential for balancing any differences between the shift registers. Specifically described is a device that compares the various offsets of a plurality of shift registers and determines a single offset potential to be applied to each shift register according to the comparison.
However, notwithstanding various calibration methods are applied, there are still other noticeable problems during calibration, for example, the memory and time for calibration. Especially, the more complex the calibration method is, the more the consumption of time and memory is. FIG. 1 is a flow chart illustrating the calibration method in accordance with the prior art. The image scanning system first captures calibration information from a calibration chart (step 110). The scanned calibration information is first saved in the calibration memory (step 120). Next, the calibration information is read and operated by the CPU of a host computer (step 130). When the calibration information is captured with multitude times, the reading and operation by the host computer may spend much time. When the host computer implements the calculation and operation for the calibration information, it may first transmit the normalized calibration information into the calibration memory (step 140). Accordingly, the calibration with the host computer spends much memory and operation time. Thus, the reduction of memory and time for calibration is very important for a high-speed scanner.